RV Batteries

They fire our engines, they light up our lives, and they ignite our curiosity.

By Bill Hendrix, F761S
January 2005

Many questions arise around the campfire when the subject of batteries is discussed. What is the best type for RV use? What brand is the best? Should we buy the most expensive battery with the best warranty, or the least expensive battery with the largest capacity?

In short, there are no concrete answers “” only opinions, and the opinions often are voiced by an expert who has a profit motive, prompting me to consider the advice with a bit of caution. I like to gather several opinions and take some of the middle ground as being fairly safe.

Starter batteries are not much of an issue. They are available in many varieties in most communities with prices paralleling their capacity and warranty. A deep-discharge battery is a different animal, not as widely available in larger sizes and often associated with confusing terminology.

When looking at the selection of starter, marine, and RV deep-cycle batteries, we are exposed to another language “” Battery Speak! The terms used in battery jargon to describe features need a little explanation:

Deep cycle refers to a battery that has heavier plate construction and denser but less porous plate paste that will tolerate an abundance of discharges below the 40 percent state of charge, and is more suitable for sustained moderate amperage loads. This is just the opposite of what is needed for a starting battery.

Cold cranking amps (CCA) and marine cranking amps (MCA) are the number of amperes available for 30 seconds at 0 degrees Fahrenheit and 32 degrees Fahrenheit, respectively. These ratings are more appropriate for selecting a starting battery, as it is designed to provide high amperage for a short period of time. If you frequently travel in your motorhome during freezing weather, select a starter battery with the best CCA.

Ampere-hour (Ah) or C/20 rating is the number of amp-hours available at 77 degrees Fahrenheit if a battery is discharged at a rate that will bring it down to 10.5 volts in a 20-hour period. (Some are rated at 80 degrees Fahrenheit.) A 100-Ah battery should be able to deliver 5 amps for 20 hours before being discharged to 10.5 volts. This rating is more suitable for RV use, as it is the only rating number applicable for mathematics to determine the useful energy available.

Reserve capacity (RC) is the amount of time, in minutes, a battery can deliver 25 amps before reaching 10.5 volts at 80 degrees Fahrenheit. (Note: This rating can be misleading, as not all battery manufacturers subscribe to this standard established by the Battery Council International.) To convert reserve capacity to ampere-hours, multiply the RC number by 0.6. Example: a battery labeled as having an RC of 150 would have approximately 90 Ah (150 X 0.6 = 90). If you make this conversion and you get an unrealistically high Ah number, that manufacturer has used a nonstandard discharge rate. I usually disregard the reserve capacity number, as I have found the multiplier to be unreliable.

State of charge (SOC) is a measurement of a battery’s available capacity expressed as a percentage of its rated capacity. This is most accurately done with a hydrometer to measure the specific gravity of the electrolyte (battery acid). However, this is not very convenient and will work only on flooded-cell batteries. The most popular and more convenient way to determine SOC is an accurate voltmeter reading of the battery at rest for open circuit voltage.

Three different types of batteries are commonly available:

A) The flooded cell, which has liquid electrolyte and a removable cap for servicing;
B) The gel cell, which has a colloidal form of gelled electrolyte in sealed cells;
C) The absorbed glass mat (AGM), also a sealed cell, with the available electrolyte being absorbed into a fiberglass material.

All are still lead-acid-style batteries. The gel cell gained immediate popularity when it was introduced several years ago as a no-maintenance battery, but it now seems to be losing popularity because of its sensitivity to overcharging.

Here are some voltage numbers for various SOC:

Note: Some charts will vary slightly from the above.

This is really good, useful information, but the bad news is that we don’t do that (disconnect the batteries and take a voltage reading). Unless we are making a battery test, we monitor the voltage as the batteries are in use and the number that we see on the voltmeter is a bit lower than a resting, open-circuit voltage during a battery test. How much lower? This will vary with the amount of the load at the time. For a light load, such as only a few lights, you could add about 0.10 to 0.15 volts to the at-work reading to approximate the state of charge.

The life cycle expectancy of deep-discharge batteries is the number of times you could expect to cycle the battery when discharging to various percentages of charge at 80 degrees Fahrenheit:

Although these figures are for batteries with flooded cell construction, the numbers for gel cell batteries should be about the same; however, the numbers for AGMs should be significantly higher. (See “Absorbed Glass Mat Batteries,” FMC, December 2000, page 62.) This chart assumes that the depth of discharge will be the same each time the batteries are used, but the point is that the deeper the batteries are discharged, the shorter their life expectancy, regardless of the type of battery.

Temperature affects battery performance in every respect. (See “Monitoring Battery Temperature,” FMC, April 1999, page 62.) Most of the rating numbers will be at temperature for either 77 or 80 degrees Fahrenheit. One battery manufacturer states that when the temperature drops to freezing, 35 percent of the battery capacity is lost, and at 0 Fahrenheit, it drops 60 percent. Another claims the loss is 3 to 4 percent for each 10 degrees below 77 degrees Fahrenheit. In either case, the reduction in available current is significant at temperatures of freezing or below. Winter sports enthusiasts should opt for sealed-cell batteries in a warm compartment for those extreme conditions to optimize performance.

Battery warranties are all over the chart. Many companies offer starter batteries with long warranty periods. The plate construction is probably a little better, but you do pay for the extra warranty period. Most of the deep-discharge batteries have a shorter warranty “” usually one to two years and often a no-charge exchange for a portion of the warranty period. I personally prefer one that provides 12 to 18 months no-charge exchange.

Popular deep-discharge battery sizes for RV use:

**Please note that the golf cart 6-volt battery must be paired in series to give 12 volts, but the amperage stays the same. Series wired, you add the voltages, and parallel wired, you add the amperages, but you can’t add both the voltage and the amperage. (See the accompanying illustration.)

When looking at the dimensions for battery groups from various suppliers, I discovered slight variations, probably due to some differences in post location, handles, and case construction. But if you are replacing a battery with one from the same group number, it should fit. The Battery Council International establishes maximum dimensions for the various battery groups, so batteries within a particular group do not all necessarily have to be the same exact size.

Another interesting fact is that for capacity rating purposes, the dead battery voltage is 10.5 volts. But the state of charge chart uses the dead battery voltage as 11.8 volts.

I also tried to make some price comparisons between a few of the more prominent suppliers. I found it to be a futile task, as there were so many different models of the same group size, and the way they were rated was not the same “” some use reserve capacity and some use amp-hours. I discovered variations in warranties as well.

Generally speaking, the flooded cell, deep-discharge battery will deliver more amp-hours for the dollar, but you must do the maintenance. The AGM will outlast others; requires no maintenance, other than external cleaning; packs more energy in the 4D and 8D sizes; and has better longevity, but this type is considerably more expensive. Battery prices are constantly on the move. Also, it is difficult to do much battery shopping on the Internet, as shipping costs are considerable, and when we need batteries, we frequently need them right now.

If you monitor your batteries, you should be able to tell when they are nearing their demise. If you try to squeeze out every last amp, you may find yourself in a dilemma that not only interrupts your trip but also may be more expensive in the end. Replace the batteries when needed, at your convenience when you have time to do some comparison shopping with no pressure.